Airsoft guns can be powered in several ways. The most basic system is powered by a compressed spring. The trigger releases the spring and allows for the airsoft BB to be shot. The most popular method of powering an airsoft rifle is via a battery. The battery stores electrical energy that is used to turn a motor that drives a piston assembly. This allows for single shot or full-auto operation. Some pistols and airsoft rifles are powered by 12 gram CO2 cartridges. CO2 is affordable and efficient, but can produce too much power for some airsoft facilities. Last, is Green gas. Green gas produces much lower velocities than CO2 and is generally considered the best fuel source for airsoft pistols and some carbines or personal defense weapon (PDW) formats.